The present invention relates to a pneumatically operated screw driver, and more particularly, to such driver capable of reducing consumption of compressed air and enhancing operability and durability.
A conventional pneumatically operated screw driver is disclosed in a Japanese Patent Application Kokai (OPI) No. Hei-1-45579. As shown in FIG. 1, the conventional driver generally includes a pneumatically rotatable rotor 1, a power transmission unit 3 driven by the rotor 1 for rotating a screw 13 about an axis thereof, a moving unit 14 slidably supported by an outer frame 7 and rotatably supporting the rotor 1 for downwardly depressing the rotating screw 13, and a screw supply unit 28 for successively supplying a screw below the power transmission unit 3.
More specifically, the conventional screw driver has the outer frame 7 in which the moving unit 14 is slidably disposed. In a hollow space of the moving unit 14, the rotor 1 is rotatably supported by support means 15 such as bearings fixed to the moving unit 14. The power transmission unit 3 including a drive bit is disposed coaxially with the rotor 1 and provided integrally therewith. Therefore, upon rotation of the rotor 1, the power drive bit is also rotated about its axis. A coil spring 11 is interposed between a lower end of the moving unit 14 and a bottom wall of the outer frame 7 for normally urging the moving unit 14 upwardly. On the bottom wall, a damper member 4 formed of a rubber is mounted to which the lower end of the moving unit 14 is abuttable for damping the moving energy of the moving unit 14.
At a lower portion of the screw driver, the screw supply unit 28 is provided. The screw supply unit 28 includes a supply piston 29, a coil spring 31, a feed pawl (not shown) and a linking band 33. The coil spring 31 is disposed within a piston chamber 29a for normally urging the supply piston 29 toward a position below the drive bit 3. The feed pawl is movably supported at a tip end of the feed piston 29 and engageable with a hole formed in the linking band 33 for supplying the screws 13 to the position immediately below the drive bit 3 in order to successively feed each one of the screws 13 to a position coaxially with the drive bit 3 and therebelow.
A fluid circuit is provided in the outer frame 7 for moving the moving unit 14 and the piston 29 and for rotating the rotor 1. That is, a compressed air inlet port 5 and outlet port 9 are provided in the frame 7. The inlet port 5 is connected to a compressed air source (not shown). These ports 5 and 9 are selectively communicatable with an operation valve 2 movable by a trigger 10 secured to the outer frame 7. A passage 16 extends from the operation valve 2. The passage 16 is in fluid communication with the piston chamber 29a for applying pneumatic pressure to the feed piston 29 to move the latter against the biasing force of the coil spring 31.
The passage 16 is also communicated with a top passage 8 for applying pneumatic pressure to a top surface of the moving unit 14 in order to move the latter downwardly against the biasing force of the coil spring 11. Further, the passage 16 is in fluid communication with an internal passage 18 within the moving unit 14 by way of a lateral bore 17 formed in a side wall of the moving unit 14 so as to introduce the compressed air into the rotor 1.
With this arrangement, when the trigger 10 is manipulated (pulled upwardly), the compressed air from the compressed air source is introduced into the passage 16 through the operation valve 2. Therefore, the compressed air is introduced into the internal passage 18 through the lateral bore 17, and reaches the rotor 1 for pneumatically rotating the same 1. Further, the compressed air is also introduced into the top passage 8 for applying pneumatic pressure at the top wall of the moving unit 14 for moving the latter downwardly against the biasing force of the coil spring 11.
By the rotation of the rotor 1, the power transmission unit 3 is also rotated in order to rotate the screw 13 about its axis, and at the same time, the screw 13 is moved downwardly by the downward movement of the moving unit 14. Thus, the screw 13 can be screwed into a workpiece 35 or a wall to be screw-fixed. During this time, the compressed air is also introduced into the screw supply unit 28, so that the feed piston 29 is moved to its retracted position (rightwardly in FIG. 1) against the biasing force of the coil spring 31.
Thereafter, if the trigger 10 is released, the compressed air supply to the top passage 8, the internal passage 18 and the screw feed unit 28 is blocked by the operation valve 2. Therefore, air in the top passage 8 is discharged outside through the outlet port 9. Accordingly, the moving unit 14 is moved upwardly by the biasing force of the coil spring 11 to restore its original position. Further, the compressed air in the screw supply unit 28 is also discharged outside through the outlet port 9, so that the feed piston 29 is moved toward the drive bit 3 by the biasing force of the spring 31. Accordingly, the screw 13 is supplied toward the position immediately below the drive bit 3.
As described above, according to the conventional screw driver, the power transmission unit 3 is provided integrally with the rotor 1, and the rotor 1 is rotatably provided within the moving unit 14. Here, generally, the rotor 1 must have a bulky size and has a heavy weight, and therefore, the moving unit 14 must also be bulky and heavy for accommodating therein the bulky rotor 1. Thus, entire weight of the conventional screw driver is almost occupied by the rotor 1 and the moving unit 14, and consequently, large reactive force may be provided at a time of screw driving operation, which may lower operability of the screw driver.
Further, due to the bulky size of the rotor 1 and the moving unit 14, the vertically moving stroke of the rotor 1 must also be large in addition to increase in diametrical direction thereof. Consequently, resultant outer frame 7 has large size, and therefore, entire screw driver must provide a bulky size, which in turn impedes operability of the screw driver.
In another aspect of the conventional screw driver, even after fixing of the screw 13 to the workpiece 35, the moving unit 14 is moved downwardly to abut against the damper member 4 due to surplus energy. In this case, large impact force may be generated due to heavy weight of the rotor 1 and the moving unit 14. This impact or repulsive force may be directly transmitted to the rotor 1 and to the rotor supporting members 15 to reduce durability or service life of the screw driver.
In still another aspect of the conventional screw driver, supply of the compressed air to the rotor 1 or a stop of the supply is performed by the opening and closing operation of the operation valve 2 by virtue of the manipulation to the trigger 10. In this case, an operator releases the trigger 10 for closing the operation valve 2 after his acknowledgement of completion of the fixing of the screw 13 with respect to the workpiece 35. Therefore, the compressed air may be uselessly supplied during a period from completion timing of the screw fixing to the release timing of the trigger 10. Accordingly, larger volume of the compressed air may be consumed.
In a recent building industry, light weight panel board such as a plaster staff is widely used, and accordingly, special types of screws for fixing such board are used in increasing amount. For this, demand has been made for using a light weight screw driver available for such screws. For the screw driving work, an electrical screw driver has been used. However, another screw driver such as a pneumatically operated screw driver having a light weight is demanded since the electrical screw driver is generally heavy. Still however, in an actual construction site, a compressor as the compressed air source must also be used in case of the employment of the pneumatically operated screw driver. The compressor must be portable and small in size, and in this connection, the above described pneumatically operated screw driver is not available, since the driver consumes large volumes of compressed air. Moreover, as described above, since the screw feed unit 28 is also pneumatically driven, the unit 28 consumes the compressed air. In view of the above, it is necessary to provide a pneumatically operated screw driver which can reduce consumption of the compressed air.